1. Field Of The Invention:
The present invention relates to methods and devices for rapidly analyzing characteristics of a sample medium, and in particular to such methods and devices for identifying characteristics of a flowing sample medium by the interaction of the sample medium with a luminescent layer.
2. Description Of The Prior Art
A method is described in German No. OS 31 48 830 for identifying at least one characteristic of a flowing sample medium by directing light of a known wavelength onto at least one luminescent layer which is directly or indirectly in contact with the sample medium. The luminescent properties of the layer change in dependence upon the characteristics of the medium, and the resulting luminescent light is supplied to detectors whose signals are a measure for the characteristic to be identified
In many instances, the measurement of the oxygen concentration in fluids (gases and liquids) must be undertaken with extremely short follow-up time of a few milliseconds, and with no "dead time" and the measurement must be undertaken such that no oxygen is consumed in the measurement. For example, the oxygen concentration in the expiration air for respiratory treatment of patients should ideally be monitored in this manner.
The principle of luminescence quenching of layers excited to luminesce with light of a defined wavelength has been used for undertaking measurements with short follow-up times. Investigations have shown, however, that the luminescent properties of such layers are influenced not only by oxygen, but also by other characteristics or components of the fluid. For example, temperature changes the intensity of the luminescent light in the same manner as oxygen, i.e., the amplitude of the emitted light decreases with increasing temperature. Moreover, increasing temperature also causes a shift toward longer wavelengths. Humidity and water also influence luminescent properties. In the specific case of respirator treatment, the influence of anesthesia gases, such as nitrous oxide or halothane, are also of consequence.
In general terms, a number of different substances exist which influence specific luminescent layers. The response of the luminescent layers to such substances generally depends upon the selected combination of carrier material and luminescent dyestuff. Flowing sample media such as gases and other fluids are employed as the sample media. This does not, however, exclude other sample media. The principle of luminescent quenching can be utilized for undertaking such measurement as long as the luminescent layer is influenced by the characteristic of the sample medium which is of interest.
The additional influences on the luminescent light by other characteristics were hitherto considered to be insurmountable difficulties for undertaking fast measurement of oxygen concentration in this manner. In order to eliminate the influence of humidity, the luminescent layers were shielded from the sample medium by a membrane as taught in U.S. Letters Patent 4,003,707. As a result, however, the follow-up time was greatly increased given the layers employed, so that this known measuring device is not suitable for rapid identification of the oxygen concentration in, for example, patient monitoring.
The influence of temperature on the luminescent layers has also been investigated, but only for the purpose of actually sensing the temperature as the characteristic of interest. The temperature effect is considered disturbing in the identification of oxygen in respiratory gases, and attempts were made to eliminate this influence by bringing the respiratory gas to a predetermined temperature before analysis thereof. Under certain conditions, a defined humidity was also set at the same time. These measures, however, also resulted in an undesirably high follow-up time.
Although luminescent layers having water repellant carrier material, whose luminescent properties are therefore not influenced by humidity, are known from the aforementioned German OS No. 31 48 830, the temperature effects nonetheless remain.